1. Field of the invention
The invention relates to a process and an apparatus for the production and/or treatment of particles.
The process and the apparatus can be used, for example, for moving particles, introduced in the solid, dry or moist state into a process chamber, in gas passed upward through the process chamber, for example to fluidize said particles and/or to move said particles on a rotor, and to apply a coating to the particles by spraying a liquid in the process chamber and/or to agglomerate the particles originally present to give larger particles--i.e. to granulate said particles by pelletization--and to dry said particles. For example, the liquid may contain at least one organic solvent and/or dispersant and/or water serving as a solvent and/or dispersant, may consist of a solution and/or contain a disperse phase and serve as a coating material or binder. If particles to be agglomerated already contain a binder present in the solid state and soluble in a solvent, it may be sufficient to spray, as the liquid, merely the stated solvent onto the particles.
However it is also possible to form the solid particles in the process chamber itself. The liquid used for this purpose may be a solution which is atomized in the process chamber into droplets, from which the solid particles form after the drying process. These may then likewise be provided with a coating and/or agglomerated.
2. Description of the prior art
U.S. Pat. No. 4,621,437 discloses fluid bed apparatuses in which a process chamber serving to receive the particles, a gas circulation device and a heat exchanger together form a gas circulation. The gas circulation has a branch which is connected to a vacuum suction pump and a condenser for solvent recovery. The process chamber contains a spray nozzle which is connected via a pump to a liquid reservoir.
During operation of these apparatuses, the gas pressure in the gas circulation is reduced by means of the vacuum suction pump to a value below atmospheric pressure and gas is circulated in the gas circulation by means of the gas circulation device. The particles present in the process chamber are fluidized by means of the circulated gas, sprayed for a certain time with a liquid containing an organic solvent and then dried without spraying of liquid.
When the spray nozzles are used, only the liquid to be sprayed but neither air nor another gas is fed to the spray nozzles of the apparatuses disclosed in U.S. Pat. No. 4,621,437. These spray nozzles thus consist of airless high-pressure nozzles. When an airless high-pressure nozzle is used, especially with intermittent spraying of a liquid coating material or binder, there is however the danger that liquid droplets in the region of the liquid outlet of the spray nozzle will remain adhering to said nozzle and dry and/or that particles from the fluid bed will adhere to the nozzle. This may influence the atomizing properties of the spray nozzle in a disadvantageous manner so that, for example, many liquid droplets are substantially larger than intended. This results in local overmoistening of particles during the coating of particles and thus promotes agglomeration, which is generally undesirable. Moreover, the spray nozzle may even be completely blocked.
From other fluid bed processes in which the particles are fluidized with a gas at approximately normal ambient air pressure, it is known per se that the liquid to be applied to the particles can be sprayed using a multi-medium nozzle. In addition to the liquid whose pressure is approximately equal to the ambient air pressure, compressed air or another compressed gas having a pressure which is greater than that of the liquid and is usually about 200 kPa to 500 kPa is supplied to said spray nozzle.
If the gas transported through the process chamber for fluidizing the particles has a pressure substantially below atmospheric pressure and, for example, consists essentially of superheated vapor of at least one organic solvent, spraying a liquid with the aid of a conventional multi-medium nozzle by supplying compressed air or other compressed gas from an external compressed gas source would have the disadvantage that, during spraying, a large amount of air or of another gas--for example nitrogen--would pass from the compressed gas source into the process chamber. This in turn would have the disadvantage that the apparatus would have to be equipped with a relatively large and expensive vacuum suction pump which would require a great deal of energy during spraying of liquid, in order to maintain in the process chamber the desired pressure below the ambient air pressure. Another particular disadvantage would be that it would be necessary to provide a large, expensive solvent recovery apparatus consuming a great deal of energy during operation, in order to liquefy the solvent vapor in the gas extracted from the circulation by the suction pump by cooling said gas and to separate said liquid from the remaining gas and to collect said liquid. If a large amount of gas which contains, for example, air or nitrogen and organic solvent vapor is fed to the solvent recovery apparatus per unit time, it is furthermore difficult completely to avoid the emergence of organic solvent into the environment or at least to keep the emerging amount of solvent sufficiently low.